The scale on a map is 1cm : 10km. If two cities are 9cm apart on the map, what is the actual distance in km between the two cities?
Solution: The scale means that every 1cm on the map corresponds to an actual distance of 10km. A distance of 9cm on the map is the same as 9 $\cdot$ 1cm. The actual distance between the two cities is 9 $\cdot$ 10km, or 90km.